Cardiac pacemaker with pacing rate monitoring

ABSTRACT

A pacing monitoring system is described for incorporation in an implantable pacemaker that monitors the pacing rate and/or cumulative pace count in order to protect a patient from excessive pacing. The system includes monitoring circuitry that is configured to operate in multiple monitoring zones, where each zone is adapted to prevent excessively high-rate pacing during a particular mode of device operation.

FIELD OF THE INVENTION

This invention pertains to methods and devices for delivering therapy tothe heart with electrical stimulation.

BACKGROUND

Cardiac rhythm management devices are implantable battery-powereddevices that provide electrical stimulation to selected chambers of theheart in order to treat disorders of cardiac rhythm. A pacemaker, forexample, is a cardiac rhythm management device that paces the heart withtimed pacing pulses. The most common condition for which pacemakers havebeen used is in the treatment of bradycardia, where the ventricular rateis too slow. If functioning properly, the pacemaker makes up for theheart's inability to pace itself at an appropriate rhythm in order tomeet metabolic demand by enforcing a minimum heart rate and/orartificially restoring AV conduction. Implantable devices may also beconfigured to treat tachyarrhythmias such as tachycardia andfibrillation with electrical stimulation. An implantablecardioverter/defibrillator (ICD) provides this kind of therapy bydelivering a shock pulse to the heart when the device detectsfibrillation. Another type of electrical therapy for tachyarrhythmias isanti-tachycardia pacing (ATP). In ATP, the heart is competitively pacedwith one or more pacing pulses in an effort to interrupt the reentrantcircuit causing a tachycardia. ATP can be applied to either theventricles or the atria. Modern ICD's typically are also pacemakers withATP capability configured so that ATP therapy is delivered to the heartwhen a tachycardia is detected, while a shock pulse is delivered whenfibrillation occurs.

The delivery of pacing pulses to heart, whether to treat bradycardia ortachycardia, is a transfer of energy that can have deleteriousphysiological effects if sustained over a long enough period and at ahigh enough rate. In order to prevent this possibility, it would bedesirable for a pacemaker to be configured to monitor the rate at whichpacing energy is transferred as well as the cumulative amount of theenergy as the pacemaker delivers pacing therapy in different modes.

SUMMARY

A pacing monitoring system is described for incorporation in animplantable pacemaker that monitors the pacing rate and/or cumulativepace count in order to protect a patient from excessive pacing. Thesystem includes monitoring circuitry that is configured to operate inmultiple monitoring zones, where each zone is adapted to preventexcessively high-rate pacing during a particular mode of deviceoperation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a cardiac rhythm management device with ATPand bradycardia pacing capability.

FIG. 2 illustrates the components of an exemplary pacing monitoringsystem.

FIG. 3 illustrates different monitoring zones according to an exemplaryembodiment.

DETAILED DESCRIPTION

The normal decision on the rate for energy delivery in an implantablepacemaker delivering bradycardia pacing is a complex combination basedon the target rate programmed by the patient's physician, detectedactivity level of the patient, and the current pacing mode of thedevice. In addition, ATP therapies to treat fast heart rates involveshort bursts of rapid pacing into either the atrium or ventricle(depending on where the fast heart rate is detected). Pacemakers mayalso be configured with other high-rate pacing modes that enable anelectro-physiologist to test the patient for certain heart conditions.Although a pacemaker could be configured to simply always limit thepacing rate below a level considered unsafe, any of the types ofpacemaker operation just mentioned may require temporary pacing at ratesthat would be hazardous if sustained for long periods. Simply disablingprotection against excessive pacing during such therapy delivery is nota desirable option since that would leave the patient vulnerable toexcessively high-rate pacing as a result of erroneous device behavior.

Disclosed herein is a pacemaker that incorporates an adaptable pacingrate monitor to protect the patient from erroneous high-rate pacing. Thepacing rate monitor utilizes separate protection zones for differentclasses of pacing therapy. Such a multiple zone approach adapts thepacing rate protection to the type of therapy being delivered byproviding separate protection zones for pacing modes that deliver energyat different rates. The criteria for acceptable pace delivery in eachzone are tailored to the therapy requirements for that zone, maximizingthe probability of detection of erroneous device behavior. For example,separate zones for high rate pacing into both the atrium and ventriclemay be provided, as the hazards associated with each are different.Patient safety is thereby maintained in the presence of device failureswhile still allowing for delivery of the desired therapy.

In an exemplary embodiment, the pacing rate monitor is capable ofoperating in four monitoring zones: a normal-rate zone, a high-ratezone, an unlimited-rate zone, and a fail-safe zone. During normalbradycardia pacing, the monitor operates in the normal-rate zone, wherea single pacing rate limit is set for both the atrium and ventriclesbased on a clinically acceptable maximum sustained pacing rate. A pacescheduled to be delivered by the pacing circuitry is blocked if thatpace would result in a pacing rate above the pacing rate limit. Inaddition, a grace interval may be provided to allow a second pace tooccur very soon after a first pace, while the heart tissue would be inrefractory, even if the pace would occur at an interval after the firstpace that would violate the pacing rate limit. No hazard exists if thesecond pace is delivered into refractory tissue which will not respondto further stimulation. Closely spaced back-up pacing is used in somenormal operating modes to ensure capture by a pacing pulse (e.g.,autocapture paces and safety paces) and are not blocked in thisembodiment. Any additional pace that occurs after the grace interval andexceeds the set high-rate limit, however, will be blocked and an errordeclared. While the device is delivering ATP therapy in this embodiment,the monitor operates in a high-rate zone that allows a higher pacingrate than the normal-rate zone and also enforces a pace count limit thatlimits the cumulative number of paces delivered over a specified periodtime. A high-rate zone may also be useful for performing certaindiagnostic testing procedures with pacing stimulation.

Separate atrial and ventricular high-rate zones may be provided fordelivering atrial or ventricular ATP therapy so that a higher rate burstof paces is permitted into either the atrium or ventricle while theother chamber is kept in the normal-rate zone. Each high-rate zone mayhave its own pacing rate limit. For example, the ventricular high-ratezone may allow pacing rates of up to 500 pulses per minute, while theatrial high-rate zone may allow pacing up to 1500 pulses per minute. Thegrace interval for closely spaced pacing remains operative for thechamber receiving pacing monitored in the normal-rate zone, while nograce interval is used in monitoring the chamber where high-ratedelivery is allowed in high-rate zone. The maximum number of pacesallowed while in either of these high-rate zones is limited to aspecified maximum count limit. Any pace that exceeds the set high-ratelimits or exceeds the count limit will be blocked and an error declared.Separate atrial and ventricular high-rate monitoring zones allow apacemaker to deliver atrial ATP therapy without creating the risk offast pacing into the ventricle which could trigger ventricularfibrillation. Fast ventricular pacing is required for ventricular ATPand is usually only provided in devices with a high-voltagedefibrillator to protect against fatal fast rhythms.

Also in an exemplary embodiment, the pacing monitor may be made tooperate in an unlimited-rate zone where no pacing rate limits or pacecount limits are enforced. The unlimited-rate zone is useful for certaintests performed under the supervision of an electro-physiologist. Inthis zone, pacing is limited to a time period (e.g., 2 seconds) afterwhich an error is declared. The time-limit of the unlimited-rate zonemay be configured to be restarted a specified maximum number of times bya manually input (i.e., via a telemetry) signal. A fail-safe zone isactivated in response to a system-reset or an error in any of the othermonitor zones. The fail-safe zone has the same characteristics as thenormal-rate zone, but has a lower pacing rate limit (e.g., 100 paces perminute). Transitions between monitoring zones are under program controlin response to certain events or to telemetry commands. Digital keys maybe used to protect against unintended transitions between zones.

1. Exemplary Pacemaker

FIG. 1 is a system diagram of a microprocessor-based cardiac rhythmmanagement device with the capability of delivering bradycardia oranti-tachycardia pacing therapy to either the ventricles or the atriaand in which may be incorporated a pacing monitor as described above.Pacemakers are usually implanted subcutaneously on the patient's chestand connected to electrodes by leads threaded through the vessels of theupper venous system into the heart. An electrode can be incorporatedinto a sensing channel that generates an electrogram signal representingcardiac electrical activity at the electrode site and/or incorporatedinto a pacing or shocking channel for delivering pacing or shock pulsesto the site.

The controller of the device is made up of a microprocessor 10communicating with a memory 12 via a bidirectional data bus, where thememory 12 typically comprises a ROM (read-only memory) for programstorage and a RAM (random-access memory) for data storage. Thecontroller could be implemented by other types of logic circuitry (e.g.,discrete components or programmable logic arrays) using a state machinetype of design, but a microprocessor-based system is preferable. As usedherein, the term circuitry should be taken to mean either theprogramming of a controller in the form of executable code stored inmemory or other storage medium or to discrete logic circuitry configuredto perform particular functions. The controller is capable of operatingthe device so as to deliver a number of different therapies in responseto detected cardiac activity. A telemetry interface 80 is also providedfor enabling the controller to communicate with an external programmeror other device via a wireless telemetry link.

The device shown in FIG. 1 has three sensing/pacing channels, where apacing channel is made up of a pulse generator connected to an electrodewhile a sensing channel is made up of the sense amplifier connected toan electrode. A MOS switch matrix 70 controlled by the microprocessor isused to switch the electrodes from the input of a sense amplifier to theoutput of a pulse generator. The switch matrix 70 also allows thesensing and pacing channels to be configured by the controller withdifferent combinations of the available electrodes. A shock pulsegenerator 90 is also interfaced to the controller for deliveringdefibrillation shocks between an electrode and the housing or can 60 asselected by the switch matrix. In an example configuration, asensing/pacing channel may include ring electrode 43 a (33 a or 23 a)and tip electrode 43 b (33 b or 23 b) of bipolar lead 43 c (33 c or 23c), sense amplifier 41 (31 or 21), pulse generator 42 (32 or 22), and achannel interface 40 (30 or 20). The channel interfaces communicatebi-directionally with a port of microprocessor 10 and may includeanalog-to-digital converters for digitizing sensing signal inputs fromthe sensing amplifiers, registers that can be written to for adjustingthe gain and threshold values of the sensing amplifiers, and registersfor controlling the output of pacing pulses and/or changing the pacingpulse amplitude. In the illustrated embodiment, the device is equippedwith bipolar leads that include two electrodes which are used foroutputting a pacing pulse and/or sensing intrinsic activity. Otherembodiments may employ unipolar leads with single electrodes for sensingand pacing which are referenced to the device housing or can 60 (oranother electrode) by the switch matrix 70. The channels may beconfigured as either atrial or ventricular channels so as to enableeither biatrial or biventricular pacing. For example, a configurationfor biventricular sensing/pacing could have one lead of a channeldisposed in the right ventricle for right ventricular sensing/pacing andanother lead of a channel disposed in the coronary sinus for leftventricular sensing/pacing.

The controller controls the overall operation of the device inaccordance with programmed instructions stored in memory, includingscheduling the delivery of paces via the pacing channels, interpretingsense signals received from the sensing channels, and implementingtimers for defining escape intervals. The sensing circuitry of thepacemaker detects a chamber sense when an electrogram signal (i.e., avoltage sensed by an electrode representing cardiac electrical activity)generated by a particular channel exceeds a specified intrinsicdetection threshold. Pacing algorithms used in particular pacing modesemploy such senses to trigger or inhibit pacing. Both bradycardia andanti-tachycardia pacing modes may be implemented in code executed by thecontroller.

Bradycardia pacing modes refer to pacing algorithms which are used topace the atria and/or ventricles in a manner that enforces a certainminimum heart rate or restores AV conduction. Bradycardia pacing modesare also used to deliver cardiac resynchronization pacing. Because ofthe risk of inducing an arrhythmia with asynchronous pacing, mostpacemakers when delivering bradycardia pacing are programmed to operatesynchronously in a so-called demand mode where sensed cardiac eventsoccurring within a defined interval either trigger or inhibit a pacingpulse. Inhibited demand pacing modes utilize escape intervals to controlpacing in accordance with sensed intrinsic activity. In an inhibiteddemand mode, a pacing pulse is delivered to a heart chamber during acardiac cycle only after expiration of a defined escape interval duringwhich no intrinsic beat by the chamber is detected. In a triggered mode,a sense occurring in one heart chamber triggers a pace to either thesame or a different heart chamber. For example, in an atrial trackingmode, an atrial sense triggers an escape interval that results in aventricular pace upon expiration.

The device of FIG. 1 may also deliver ATP therapy and be programmed witha plurality of selectable ATP pacing protocols that define the manner inwhich anti-tachycardia pacing is delivered. In a microprocessor-baseddevice, the output of pacing pulses is controlled by a pacing routinethat implements the selected pacing protocol as defined by variousparameters. A data structure stored in memory contains the parametersets that define each of the available pacing protocols. Pacingprotocols for ATP therapy attempt to block the reentrant depolarizationwavefront causing the tachycardia with depolarizing wavefronts producedby a burst of pacing pulses. Protocols may vary according to parametersthat define the number of pulses delivered and the particular timingemployed. For example, the protocol may define a burst of pulsesdelivered at a specified pacing interval (or with varying pacingintervals) and for a specified time. The protocol may further define theduration and amplitude of the pacing pulses. Different protocols are aptto be more successful than others in terminating particulartachyarrhythmias that may differ as to rate and/or depolarizationpattern. For this reason, modern cardiac rhythm management devices arecapable of employing a number of different ATP protocols to delivertherapy, all of which involve the delivery of pacing pulses at a rapidrate for a short period of time.

2. Pacing Rate Monitoring System and Operation

FIG. 2 shows the components of an exemplary pacing monitoring systemthat may be incorporated into an implantable pacemaker such as shown inFIG. 1. The circuitry to be described may be implemented as codeexecuted by microprocessor-based controller, as discrete circuitcomponents, or as some combination thereof. For example, in oneembodiment, the monitoring circuitry 201 is a separate circuit componentin communication with and under the control of the microprocessor-basedcontroller. The pacing control circuitry 200 schedules atrial andventricular paces (A-paces and V-paces, respectively) in accordance witha bradycardia pacing algorithm, an ATP protocol, or in response tocommands received via telemetry. When a pace is scheduled by the pacingcontrol circuitry, an A-pace or V-pace command is issued to themonitoring circuitry 201 which, if the scheduled pace is not to beblocked, then passes the command to pacing circuitry 204. Pacingcircuitry 204 represents those components of the device that actuallygenerate and deliver a pacing pulse to heart, either as an A-pace orV-pace. Monitoring circuitry 201 also receives inputs from pace countingcircuitry 202 and interval detection circuitry 203. Pace countingcircuitry 202 maintains a count of the total number of atrial andventricular paces delivered by the pacing circuitry 204 over a specifiedperiod of time or since a specified event such as a change in pacingmode. Other embodiments may maintain separate pace counts for differentheart chambers. Interval detection circuitry measures the time intervalfrom the last pace delivered by the pacing circuitry 204 for atrialand/or ventricular paces. The reciprocal of the intervals from the lastatrial or ventricular paces to next scheduled atrial or ventricular pacerepresents the instantaneous atrial or ventricular pacing rate,respectively, were the scheduled pace to be delivered. Other embodimentsmay measure intervals since the last pace for other heart chambers(e.g., right and left ventricles) to derive separate pacing rates forthose chambers. Based upon the pace count and pacing intervals receivedfrom the counting and interval detection circuitry, the monitoringcircuitry makes a pacing decision whether to block the scheduled pace orpass the pace command to the pacing circuitry 204. As described below,the pacing decision is made in a manner that depends upon the monitoringzone that the monitoring circuitry is operating in.

In order to provide the pacemaker with protection against erroneoushigh-rate pacing and/or excessive delivery of pacing energy to theheart, the monitoring circuitry is designed to operate in two or moremonitoring zones. One of the zones is the normal-rate zone for useduring normal bradycardia pacing. When operating in the normal-ratezone, the monitoring circuitry is configured to set an error flag if thepacing rate exceeds a specified normal-rate limit value. As used herein,an error flag is any signal or indication to signify that the monitoringcircuitry has detected an error condition. Another monitoring zone isthe high-rate zone for use during ATP pacing, as well as certain testmodes where bursts of high-rate pacing are employed. When operating inthe high-rate zone, the monitoring circuitry is configured to set anerror flag if either the pacing rate exceeds a specified high-rate limitvalue or if the pace count exceeds a specified count limit value. In thenormal-rate zone, the monitoring circuitry may be configured to blockthe delivery of a scheduled pace that would exceed the normal-rate limitvalue and thereby causes the setting of an error flag. Similarly, in thehigh-rate zone, the monitoring circuitry may be configured to blockdelivery of a scheduled pace that would exceed the high-rate limit valueor the count limit value and thereby causes the setting of an errorflag. The monitoring circuitry in the normal-rate zone may be configuredto permit, without setting an error flag, delivery of a pace during aspecified grace interval after the preceding pace without regard to thespecified rate limit value. The device may also incorporate circuitry tocause the monitoring circuitry to automatically operate in thenormal-rate zone if pacing is being delivered in a bradycardia pacingmode and to operate in the high-rate zone if the pacing is beingdelivered in an anti-tachycardia pacing mode.

In one particular embodiment, where the device is configured to deliverATP or other high-rate pacing to either the atria or ventricles, themonitoring circuitry is capable of simultaneously operating in either anatrial high-rate zone and a ventricular normal-rate zone or aventricular high-rate zone and an atrial normal-rate zone. Whenoperating in the atrial high-rate zone and ventricular normal-rate zone,the monitoring circuitry sets an error flag if either 1) the atrialpacing rate exceeds a specified atrial high-rate limit value or if thepace count exceeds the specified count limit value or 2) if theventricular pacing rate exceeds the normal-rate limit value. Similarly,when operating in the ventricular high-rate zone and atrial normal-ratezone, the monitoring circuitry sets an error flag if either 1) theventricular pacing rate exceeds a specified ventricular high-rate limitvalue or if the total pace count exceeds the specified count limit valueor 2) the atrial pacing rate exceeds the normal-rate limit value. Thedevice may also include circuitry to cause the monitoring circuitry tooperate in the normal-rate zone if pacing is being delivered in abradycardia pacing mode and to cause the monitoring circuitry to operatein the atrial high-rate zone and the ventricular normal-rate zone if thepacing is being delivered in an atrial anti-tachycardia pacing mode andin the ventricular high-rate zone and atrial normal-rate zone if thepacing is being delivered in a ventricular anti-tachycardia pacing mode.The monitoring circuitry may also be configured to block delivery of ascheduled pace that would exceed the normal-rate limit value, the atrialhigh-rate limit value, the ventricular high-rate limit value, or thespecified count limit value and that results in the setting of an errorflag.

In another particular embodiment, the pacing circuitry of the device iscapable of delivering pacing pulses to first and second heart chambersin accordance with a programmed pacing mode, where the first and secondheart chambers could be the right and left ventricles (or atria). Theinterval detection circuitry is configured to measure the intervalbetween the times of a scheduled pace and a preceding delivered pace forboth first chamber and second chamber paces and derive a first chamberpacing rate and a second chamber pacing rate therefrom. The countingcircuitry is configured to count the number of first chamber and secondchamber pacing pulses delivered over a specified time period and derivea total pace count therefrom (which may also include atrial paces). Themonitoring circuitry is then configured to operate in either anormal-rate zone or a high-rate zone for both chambers.

In another embodiment, a fail-safe monitoring zone is provided inaddition to normal-rate and high-rate zones, where the monitoringcircuitry transitions to the fail-safe zone whenever an error flag isset is one of the other zones. The fail-safe zone has a specifiedfail-safe rate limit value, and the monitoring circuitry preventsscheduled paces from being delivered above the specified fail-safe ratelimit value when operating in the fail-safe rate zone. The monitoringcircuitry may be made further capable of operating in an unlimited-ratezone in which no rate limit values or count limit values are imposedthat may be used for electrophysiological testing. The monitoringcircuitry is caused to transition to the unlimited-rate zone uponreceipt of an unlimited-rate zone command by the telemetry circuitry ofthe device and remains in the unlimited-rate zone for a specified testduration (e.g. 2 seconds). An error flag is set after the specified testduration expires at which point the monitoring circuitry reverts to thefail-safe zone. In a particular embodiment, the monitoring circuitryremains in the unlimited-rate zone for as long as an unlimited-rate zonecommand is continually received up to a specified clinical duration thatis greater than the specified test duration (e.g., 30 seconds). Forexample, the unlimited-rate command may be issued from an externalprogrammer when a clinician presses a particular key or button. As longas the key is depressed, an unlimited-rate zone command is continuallytransmitted so that the monitoring circuitry remains in theunlimited-rate zone until the either the key is released or thespecified clinical duration expires.

FIG. 3 illustrates the monitoring zones just described that themonitoring circuitry is configured to operate in according to oneembodiment: a normal-rate zone Z1, a ventricular high-rate zone Z2 v, anatrial high-rate zone Z2 a, an unlimited-rate zone Z3, and a fail-safezone Z4. As shown in the figure, the circuitry transitions from thenormal-rate zone to the fail-safe zone upon occurrence of an intervalviolation, and transitions from the atrial or ventricular high-rate zoneto the fail-safe zone upon occurrence of either an interval violation ora pace count violation. A time violation causes the monitoring circuitryto transition from the unlimited-rate zone to the fail-safe zone. Thedevice may be configured so that the monitoring circuitry is caused totransition to any selected zone upon receipt of a zone select transitioncommand via telemetry and/or is automatically caused to transition toappropriate zone or zones depending upon the type of pacing beingdelivered.

Although the invention has been described in conjunction with theforegoing specific embodiments, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

1. A cardiac rhythm management device, comprising: pacing circuitry todeliver pacing pulses; control circuitry to schedule paces to bedelivered by the pacing circuitry in accordance with a programmed pacingmode; interval detection circuitry to measure the interval between thetimes of a scheduled pace and a preceding delivered pace, the reciprocalof the interval being the pacing rate; counting circuitry to count thenumber of pacing pulses delivered over a specified time period andderive a pace count therefrom; monitoring circuitry to monitor thepacing rate and the pace count, wherein the monitoring circuitry iscapable of operating in either a normal-rate zone or a high-rate zone;wherein, when operating in the normal-rate zone, the monitoringcircuitry is configured to set an error flag if the pacing rate exceedsa specified normal-rate limit value; and, wherein, when operating in thehigh-rate zone, the monitoring circuitry is configured to set an errorflag if either the pacing rate exceeds a specified high-rate limit valueor if the pace count over the specified period of time exceeds aspecified count limit value.
 2. The device of claim 1 wherein themonitoring circuitry blocks delivery of a scheduled pace that wouldexceed the normal-rate limit value, the high-rate limit value, or thecount limit value and that results in the setting of an error flag. 3.The device of claim 2 further comprising circuitry to cause themonitoring circuitry to operate in the normal-rate zone if pacing isbeing delivered in a bradycardia pacing mode and to cause the monitoringcircuitry to operate in the high-rate zone if the pacing is beingdelivered in an anti-tachycardia pacing mode.
 4. The device of claim 1wherein: the pacing circuitry is capable of delivering pacing pulses tofirst and second heart chambers in accordance with a programmed pacingmode; the interval detection circuitry is configured to measure theinterval between the times of a scheduled pace and a preceding deliveredpace for both first chamber and second chamber paces, the reciprocal ofthe intervals being the first chamber pacing rate and the second chamberpacing rate; the counting circuitry is configured to count the number offirst chamber and second chamber pacing pulses delivered over aspecified time period and derive a total pace count therefrom; and, themonitoring circuitry is further capable of operating in either anormal-rate zone or a high-rate zone for both of the first and secondchambers.
 5. The device of claim 4 wherein the first and second chambersare the right and left atria.
 6. The device of claim 4 wherein the firstand second chambers are the right and left ventricles.
 7. The device ofclaim 1 wherein: the pacing circuitry is capable of delivering atrialand ventricular pacing pulses in accordance with a programmed pacingmode; the interval detection circuitry is configured to measure theinterval between the times of a scheduled pace and a preceding deliveredpace for both atrial and ventricular paces, the reciprocal of theintervals being the atrial pacing rate and the ventricular pacing rate;the counting circuitry is configured to count the number of atrial andventricular pacing pulses delivered over a specified time period andderive a total pace count therefrom; the monitoring circuitry is furthercapable of simultaneously operating in either an atrial high-rate zoneand a ventricular normal-rate zone or a ventricular high-rate zone andan atrial normal-rate zone; the monitoring circuitry, when operating inthe atrial high-rate zone, is configured to set an error flag if eitherthe atrial pacing rate exceeds a specified atrial high-rate limit valueor if the pace count over the specified period of time exceeds thespecified count limit value; and, the monitoring circuitry, whenoperating in the ventricular high-rate zone, is configured to set anerror flag if either the ventricular pacing rate exceeds a specifiedventricular high-rate limit value or if the total pace count over thespecified period of time exceeds the specified count limit value.
 8. Thedevice of claim 7 wherein the monitoring circuitry blocks delivery of ascheduled pace that would exceed the normal-rate limit value, the atrialhigh-rate limit value, the ventricular high-rate limit value, or thespecified count limit value and that results in the setting of an errorflag.
 9. The device of claim 7 further comprising circuitry to cause themonitoring circuitry to operate in the normal-rate zone if pacing isbeing delivered in a bradycardia pacing mode and to cause the monitoringcircuitry to operate in the atrial high-rate zone and the ventricularnormal-rate zone if the pacing is being delivered in an atrialanti-tachycardia pacing mode and in the ventricular high-rate zone andatrial normal-rate zone if the pacing is being delivered in aventricular anti-tachycardia pacing mode.
 10. The device of claim 2wherein the monitoring circuitry is capable of operating in a fail-safezone having a specified fail-safe rate limit value, wherein thefail-safe zone is transitioned to when an error flag is set, and whereinthe monitoring circuitry prevents scheduled paces from being deliveredabove the specified fail-safe rate limit value if the fail-safe ratezone is in operation.
 11. The device of claim 10 wherein the monitoringcircuitry is capable of operating in an unlimited-rate zone in which norate limit values or count limit values are imposed.
 12. The device ofclaim 11 further comprising: telemetry circuitry to receive telemetrycommands; wherein the monitoring circuitry is caused to enter theunlimited-rate zone when the device receives an unlimited-rate zonecommand via telemetry and remains in the unlimited-rate zone for aspecified test duration; and, wherein an error flag is set after thespecified test duration expires at which point the monitoring circuitryreverts to the fail-safe zone.
 13. The device of claim 12 wherein themonitoring circuitry remains in the unlimited-rate zone for as long asan unlimited-rate command is continually received up to a specifiedclinical duration that is greater than the specified test duration. 14.The device of claim 12 wherein the monitoring circuitry is caused totransition to any of the normal-rate, high-rate, unlimited-rate, orfail-safe zones when the device receives a zone select transitioncommand via telemetry.
 15. The device of claim 2 wherein the monitoringcircuitry in the normal-rate zone permits, without setting an errorflag, delivery of a pace during a specified grace interval after thepreceding pace without regard to the specified rate limit value.